Reduction in nitrogen oxide levels in exhaust gases both from stationary and from mobile combustion systems, especially in motor vehicles, is accomplished using the known method of selective catalytic reduction (SCR). This involves reducing nitrogen oxides in the presence of ammonia and oxygen to nitrogen. Various catalytic converter types and systems are known in principle for the acceleration of this reaction. An established class of catalytic converter is based on a titanium-vanadium catalytic converter system. Titanium-vanadium catalytic converters of this kind are used both in stationary systems and in mobile combustion systems. A further class of catalytic converter which has recently been at the focus of attention, especially for mobile use in motor vehicles, is that of zeolite-based catalytic converters. These catalytically active components include, more particularly, what are called iron- or copper-exchanged zeolites, preferably of the ZSM-5 (MFI), beta (BEA) or chabazite (CHA) type.
The catalytic converters used nowadays in motor vehicles are predominantly ceramic honeycomb catalytic converters. In operation, the exhaust gas to be cleaned flows through channels in the catalytic converter which has been extruded, for example.
A basic distinction is drawn here between what are called all-active extrudates and coated supports, known as “washcoats”. In the all-active extrudates, a catalytically active catalyst material forms the extruded body, meaning that the channel walls of the catalytic converter are formed completely from a catalytically active material. In the washcoats, a catalytically inert, extruded support body is coated with the actual catalytically active catalyst material. This is effected, for example, by dipping the extruded support body into a suspension comprising the catalyst material.
In principle, the aim is a maximum catalytic activity in the catalytic converters, i.e. a maximum NOx conversion.
A crucial factor for a maximum NOx conversion is very good contact between the offgas to be cleaned and the catalytically active material. The catalytic conversion proceeds to a crucial degree in the near-surface region at the walls of the respective flow channel through which the exhaust gas flows. Especially in the case of all-active extrudate honeycomb catalysts in which the entire extruded body is comprised of catalytically active material, the effect of this is that comparatively large volume regions of the catalyst material remain unutilized for the NOx conversion.
In the light of this, it is an object of the invention to specify a catalytic converter, especially an SCR catalytic converter, with improved catalytic activity.